Mildewed feed given to cows was the cause of the excessive levels of aflatoxin found in milk from Mengniu Dairy Group and Fujian Changfu Dairy Industry Group, according to China's quality supervision and inspection agency.

The dairies were ordered to destroy the tainted milk by the agency, which said that the contamination would end once cows stopped eating the rotten feed.

Aflatoxin is produced by a fungus that commonly grows on grain and legume crops such as peanuts, soy, corn and wheat. The toxin turns up in the milk of animals that eat affected crops. Though at low doses it is not considered harmful to humans, high doses are linked to cancer, especially in the liver.

Both Mengniu and Changfu have issued public apologies. Mengniu said that the tainted products were produced at a subsidiary in Sichuan province and none had entered the market. Changfu said it recalled the affected products immediately after inspectors told the company.

In the worst scandal, at least six infants died and 300,000 children were made ill in 2008 from drinking infant formula and milk products made with melamine, an industrial chemical that was being added to watered-down milk to elevate protein levels in quality tests.